Cross-reference is made to a copending U.S. patent application Ser. No. 484,184 by Peter W. Jensen filed on June 28, 1978 concurrently with the present application and entitled "Synchronous Color Conversion System".
This invention concerns systems for eliminating timing errors from color television signals and particularly from color television signals having incoherent components.
Television signals are composite signals carrying monochrome and color information and synchronizing waveforms. The synchronizing waveforms repeat at known periodic intervals and include horizontal line pulses, vertical field pulses and color burst. The monochrome information is obtained in a luminance component and the color information in a chrominance component. When the synchronizing waveforms exhibit phase or frequency deviations with respect to a stable reference signal of the same type, the difference in timing between these signals is a so-called time-base error. If time-base errors exist, distortion of the television picture follows. Time-base errors are formed in systems for reproduction of composite television signals recorded on a storage medium, such as magnetic tape. These errors are inherent to the process of translating the signals onto and off the tape. Many prior art devices have been developed which deal with the correction of time-base errors in reproduction of television signals.
A large group of prior art color television signal reproducing systems phase-locks the chrominance signal to a stable reference subcarrier without fully correcting the timing errors of the composite color television signal. The output signal of these reproducing systems is stabilized as to the color hue and saturation. But, the remaining timing errors inherent in the recording and subsequent reproducing process are unacceptable for some television signal applications.
As examples of the above-type prior art video recorders, the following systems may be mentioned: INSTAVIDEO video tape recorder (VTR) manufactured by AMPEX Corporation assignee of the present application, SONY VO-1600 VTR, SONY AV-8400 VTR. These systems employ the heterodyne signal processing technique, such as standardized by the Video Tape Recording Committee of the Electronic Industries Association of Japan (EIAJ).
For convenience, throughout the specification it will be referred to the above type prior art video tape recorders as "EIAJ type" recorders.
For better understanding of the invention, a brief description of an INSTAVIDEO recording and reproduce system, representing a prior art EIAJ-type video tape recorder/reproduce system follows.
In the recording part of an INSTAVIDEO VTR the transmitted composite color television signal is separated and formed into a frequency-modulated (fm) luminance component and frequency-transposed chrominance component and then both components are combined and recorded.
In the reproduce part of the INSTAVIDEO VTR, the chrominance component, which contains the color information and the color burst synchronizing waveform is separated from the luminance component by filters. Then, the original transmission frequency of both components is reconstituted for reproduction. At the frequency-reconstitution, a stable frequency chrominance subcarrier reference signal is utilized to correct the chrominance component in the following manner. First, the separated chrominance component is frequency-converted to a standard 3.58 MHz nominal carrier frequency. The color burst is extracted from the separated and frequency-converted chrominance component and it is phase-compared in a phase detector with the stable frequency reference signal from a crystal oscillator set at 3.58 MHz. The resulting phase error-voltage, which is representative of color errors, is employed to control a voltage-controlled variable frequency oscillator whose output frequency variations are responsive to the detected phase error. The output signal of the voltage-controlled oscillator is then frequency-converted to a higher frequency band. This high-band signal is in turn utilized for the above-mentioned frequency conversion of the separated chrominance component to a standard 3.58 MHz nominal carrier frequency. The detection of phase errors and concomitant phase adjustment of the voltage-controlled oscillator affect elimination of phase deviations of the color burst and, consequently, of the chrominance signal subcarrier with respect to the stable reference signal. The resulting signal is a frequency-reconstituted color-corrected chrominance component.
The fm luminance component of the composite television signal, which has been separated from the chrominance component in the reproduce part of the VTR, is first frequency demodulated to obtain its original transmission frequency band and then delayed to compensate the delay of the chrominance component due to the color-correction process. The uncorrected and delayed luminance component is then recombined with the color-corrected chrominance component for output.
As a result of the record and the reproduce process employed in the INSTAVIDEO tape recorder/playback system, the luminance and chrominance components are permitted to become and remain incoherent with respect to each other. Thus, the recombined components often contain time-base errors inherent in the recording and subsequent reproduce process. These timing errors change arbitrarily during one field of the reproduced television picture in either direction.
Analogously to the INSTAVIDEO, other prior art EIAJ-type tape recorder systems exhibit similar time-base errors. Applications in which high quality reproduction of the television signal is required need time-base correction of both chrominance and luminance component. Commonly, such correction is done by conventional time-base correctors (TBC), which perform line-to-line correction of the reproduced signal.
U.S. Pat. No. 3,763,317, assigned to the assignor of the present patent application, describes a TBC representative of existing conventional systems performing time-base corrections of composite color television signals. Such time-base correctors perform line-to-line corrections of the recorded television signal by repositioning in time each horizontal line, consequently, each component of the television signal, such as the color burst, the video information, etc. The corrections are made relative to line reference sync pulses and a chrominance subcarrier reference signal developed by independent reference sources. This is achieved by a delay device including a plurality of delay lines having binary ordered delay periods which are selectively combined to form a composite delay for each successive cycle of the repetitive signal. The combination of delay lines is selected according to comparisons of the line sync pulse and color burst of the line being corrected with the corresponding reference signals provided by the reference sources so that each line of signal information is synchronized with a line reference sync pulse.
Such time-base correctors assume coherency of the signal's horizontal sync and color burst components. It would be desirable to interface an EIAJ type color video recorder with a conventional line-to-line time-base corrector of the above-described type to achieve an improved quality of the reproduced signal from the EIAJ type recorder.
However, the output signal from these recorders is not suitable for correction by the TBC system for the following reasons:
The variable delay device of the time-base corrector is designed to develop a time delay corresponding to phase deviations of the composite television signal's color burst component ranging from 0.degree. to 360.degree.. If the phase deviation of the color burst between successive lines of the signal exceeds 360.degree. in either direction, the delay provided by the delay device is advanced or retarded one-full cycle of the subcarrier, depending on the direction in which the deviations of successive lines change. This requires the luminance and chrominance component of the television signal to be coherent, otherwise this one-cycle delay introduces an ambiguous timing relationship with respect to the horizontal sync, resulting in a jitter on the television screen whenever the phase deviation passes through 0.degree. or 360.degree..
From the above description it follows that the deficiencies of the prior-art EIAJ type color television recorders cannot be overcome by interfacing them with existing line-to-line time-base correctors in order to achieve high quality signal reproduction. Instead, when these two systems are interfaced, additional timing errors of the video signal component in the form of jitter are introduced.